Pattern matching custom implementation of List ADT - scala

Question

I understand (kind of) how pattern matching works in Scala,
Let's say I have two lists of the form:
sealed abstract class IntList
case class Empty() extends IntList // The empty list, often called Nils
case class Element(n: Int, tail: IntList) extends IntList // Element is usually called Cons
Let's say I want to create the function take(n, xs)
It should return the first n elements of xs.
I tried with normal pattern matching:
def take(n: Int, xs: IntList): IntList = xs match {
case n == 0 => Empty()
case xs : Empty => Empty()
case xs : Element => Element(xs.n, take(n-1, xs))
}
But then of course, n is not recognised, error: not found: value == case n == 0 => Empty()
How can I do this, it is probably simple but I am a beginner in Scala?

Answer 1

You have a typo in the first case where it should be
case _ if n == 0 => Empty()
and a bug in third case where you forgot to pass the tail
case xs: Element => Element(xs.n, take(n-1, xs.tail))
Try
sealed trait IntList
case object Empty extends IntList
case class Element(n: Int, tail: IntList) extends IntList
def take(n: Int, xs: IntList): IntList = xs match {
case _ if n == 0 => Empty
case Empty => Empty
case Element(n, tail) => Element(n, take(n-1, tail))
}
val list = Element(1, Element(2, Element(3, Empty)))
take(2, list) // res0: IntList = Element(1,Element(2,Empty))
Consider case object instead of case class Empty() when there is no data as per Differences between case object T and case class T() when defining ADT?